U.S. patent number 5,508,605 [Application Number 08/248,113] was granted by the patent office on 1996-04-16 for method for measuring rf pulse frequency.
This patent grant is currently assigned to AlliedSignal Inc.. Invention is credited to Elliott J. Greene, Pei-Hwa Lo.
United States Patent |
5,508,605 |
Lo , et al. |
April 16, 1996 |
Method for measuring RF pulse frequency
Abstract
A method for measuring the frequency of a stream of RF pulses
using multi-purpose, commercial-off-the-shelf test devices, such as
an RF signal down converter, a digitizer and a signal processor.
The method is based on digital signal processing and determining
the zero-crossings of the signal using signal interpolation of the
pulse points. The method is suitable for real-time calculations of
the frequency.
Inventors: |
Lo; Pei-Hwa (Ramsey, NJ),
Greene; Elliott J. (Union, NJ) |
Assignee: |
AlliedSignal Inc. (Morris
Township, NJ)
|
Family
ID: |
22937729 |
Appl.
No.: |
08/248,113 |
Filed: |
May 24, 1994 |
Current U.S.
Class: |
324/76.42;
708/290; 324/76.44; 324/76.58; 702/75 |
Current CPC
Class: |
G01R
23/165 (20130101); G01R 23/14 (20130101); G01S
7/021 (20130101); G01S 7/2883 (20210501) |
Current International
Class: |
G01R
23/00 (20060101); G01R 23/16 (20060101); G01S
7/02 (20060101); G01R 23/165 (20060101); G01R
23/14 (20060101); G01S 7/288 (20060101); G01S
7/285 (20060101); G01S 005/02 () |
Field of
Search: |
;73/861.25
;324/76.47,76.55,76.42,76.41 ;364/577,484,723
;342/192,432,442,147 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wieder; Kenneth A.
Assistant Examiner: Solis; Jose M.
Attorney, Agent or Firm: Kreger, Jr.; Verne E.
Claims
What is claimed is:
1. A method for measuring the carrier frequency of a stream of RF
pulses comprising:
(a) down converting said RF pulses to a pulse stream having an
intermediate carrier frequency by taking the difference between the
carrier frequency of said RF pulses and the carrier frequency of a
control frequency;
(b) digitizing said pulse stream;
(c) extracting pulse points and baseline data of a sample from said
pulse stream;
(d) subtracting said baseline data from said pulse points;
(e) locating initial points in time before and after each zero
crossing within a time interval, subdividing said time interval and
repetitively interpolating said initial points using Sin(X)/X to
find time points before and after a zero crossing within a
specified time accuracy range;
(f) calculating the period of the carrier frequency of said pulse
stream signal from said determined zero-crossing times;
(g) calculating the carrier frequency of said pulse stream signal
from said calculated period; and
(h) calculating the RF pulse stream carrier frequency by summing
said calculated intermediate carrier frequency with said control
carrier frequency.
2. The method of claim 1 further comprising repeating steps (a)
through (h) on other pulse streams collected in sequence and
averaging the frequency values obtained in step (h) for all of the
pulse streams.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pulse RF signals, and more
particularly to an inexpensive method of measuring the RF carrier
frequency of a stream of pulse RF signals in real-time or near
real-time.
2. Description of Related Art
Radar and electronic counter measure (ECM) systems commonly employ
pulse KF signals with narrow pulse widths and low duty cycles. Such
pulses typically are generated by on/off switching (e.g. using a
pin switch diode) of a continuous RF source, and can be of
extremely short duration (100 nanoseconds). The frequency of an RF
signal emitted by these systems is an important characteristic that
must be accurately measured to assess a system's performance.
The conventional method for measuring RF frequency is to use
expensive equipment such as electronic counters, spectrum
analyzers, synchronous detectors and digital frequency
discriminators, among others. The microwave counters EIP 1230A and
1231A manufactured by EIP Microwave, Inc. and the Hewlett Packard
electronic counter HP 5361B are examples of instruments currently
in use to measure RF pulse frequencies. These methods, however, do
not provide the required frequency measurement accuracy for
state-of-the-art radar and ECM systems. Electronic counters can
provide only 500 Khz accuracy for a 100 ns pulse width signal. The
spectrum analyzer accuracy for a 100 ns pulse width is typically
150 Khz. Additionally, electronic counters and spectrum analyzers
are expensive, bulky, and require specialized hardware. Synchronous
detectors are limited to 10 Khz measurement accuracy for a 100 ns
pulse width signal while digital frequency discriminators can
provide only 100 Khz measurement accuracy for a 100 ns pulse width
signal. However, as the radar and ECM systems become more advanced,
test technology must improve in order to assure the proper system
performance.
Furthermore, in order to characterize the performance of a Unit
Under Test (UUT) in an end-to-end test environment, one should not
have to access the internal structure of the UUT
The present invention overcomes the aforementioned
disadvantages.
SUMMARY OF THE INVENTION
The present invention is a real-time method that is able to measure
the frequency of a RF pulse down to 100 nanoseconds in width with
very high accuracy (less than 30 kilohertz) at RF signal
frequencies up to 40 gigahertz.
According to the present invention, the foregoing and other objects
are attained by using multi-purpose, commercial-off-the-shelf test
devices, such as an RF signal down converter, a digitizer and a
signal processor. The method for measuring the carrier frequency of
a stream pulses comprises the steps of extracting pulse points and
baseline data of a sample of the RF signal; determining the
zero-crossings of the signal using signal interpolation of the
pulse points and baseline data; calculating the period of the pulse
stream signal from the determined zero-crossings; and calculating
the frequency of the pulse stream signal from the calculated
period.
In one embodiment the RF signal is mixed with a control frequency,
filtered and then digitized. Preferably, data from two hundred
pulses is collected in sequence, mixed and digitized to improve the
the frequency calculation accuracy. A measurement algorithm is then
applied to the digitized data which includes extracting pulse
points and the baseline, removing DC component by subtracting the
baseline from pulse points, zero interlacing the pulse points,
applying a frequency analysis, filtering in a frequency domain and
a second frequency analysis. Zero-crossings are then calculated
using any of a number of interpolation techniques including linear
interpolation, Sin(X)/X interpolation and polynomial interpolation.
The period of the waveform is then calculated, from which the
frequency is calculated.
In an alternate embodiment the method uses Sin(X)/X interpolation
of the pulse points and baseline to determine zero-crossing points.
An iterative search algorithm finds the zero-crossing points to an
arbitrary accuracy of between about 1/100 to about 1/1,000,000. The
period of the waveform is then calculated, from which the frequency
is calculated.
An object of this invention is to improve the accuracy of RF pulse
frequency measurements in real-time or near real-time, using only
the RF output of the system under test as the input to the
invention.
A further object of this invention is to provide an accurate RF
pulse frequency measurement technique using low cost
commercial-off-the-shelf (COTS) test equipment.
An advantage of this invention is that it samples only a small
number of RF pulses thereby reducing measurement time.
Another advantage of this invention is that it is able to
characterize some frequency agile pulsed signals in which the
carrier frequency varies over time.
A further advantage of this invention is that it is easy to
maintain and to operate.
Additional objects, advantages and novel features of the invention
will be set forth in part in the description which follows, and in
part will become apparent to those skilled in the art upon
examination of the following or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and advantages of this invention will
be apparent on consideration of the following detailed description,
taken in conjunction with the accompanying drawings, in which like
reference characters refer to like parts throughout, and in
which:
FIG. 1 is a block diagram of the RF pulse signal acquisition
procedure;
FIG. 2 is a depiction of a digitized sinusoidal signal;
FIG. 3 is a flow diagram of one embodiment of the invention
and;
FIG. 4 is a flow diagram of an alternate embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
The following detailed description, which describes only the
preferred embodiments of the invention in detail, is understood
only to be an illustration of the best mode contemplated of
carrying out the invention. As will be realized, the invention is
capable of other and different embodiments, and its several details
are capable of modifications in various obvious respects, all
without departing from the invention. Accordingly, the drawings and
description are to be regarded as illustrative in nature, and not
as restrictive.
FIG. 1 illustrates the signal acquisition mechanism of the present
invention. The input pulses generated by the UUT are frequency down
converted to a pulse stream having an intermediate frequency (IF).
Mixer 22 receives the test signal from source 20 and a control
frequency generated by a local oscillator 24 and outputs an IF
signal as the difference between their frequencies (e.g.
Frequency.sub.RF -Frequency.sub.control =IF). The IF signal passes
through bandpass filter 26 to remove unwanted high frequency
components. The IF signal is sampled by a digitizer 28 and
transferred to a signal processor 30 to perform frequency analysis.
The preferred IF is approximately from 20 to 30% of the digitizer
sampling frequency. Preferably, the sampling frequency of the
digitizer chosen should allow for at least two cycles of the down
converted test signal (i.e. the IF signal) to be present within
each acquired pulse of the test signal. For example, given a 100
nanosecond pulse, digitized at 100 megahertz, preferably, the IF
frequency should be approximately 25 megahertz. Such a
configuration would result in 2.5 cycles of the IF signal being
digitized as shown in FIG. 2. In order to improve frequency
estimation accuracy, data from two hundred pulses are collected in
sequence.
The acquired data of FIG. 2 serves as the input to the pulse
frequency measurement algorithm, shown in FIG. 3. The method
interpolates the signal by first extracting pulse points and the
baseline data 32. Pulse points refer to points within the RF pulse,
and baseline points surround the pulse points where the RF signal
is not present. The baseline is extracted and subtracted from the
pulse points in order to remove any DC component from the pulse
sine wave, thereby improving accuracy of the zero-crossings. The
method then provides for zero interlacing the pulse points 34 of
the input signal by adding zeros between the extracted points. A
preferred range of an interlacing ratio is between 2 and 10. The
most preferred interlacing ration is 4. Additional functions
include applying a forward Fast Fourier Transform (FFT) frequency
analysis 36, filtering in a frequency domain using a Gaussian
filter 38, and then applying an inverse FFT on the filtered
frequency domain data 40.
The resultant data is then used to determine zero-crossings via
signal interpolation 42. This method provides high quality image
signal suppression as well as smooth transient response. Possible
interpolation techniques include linear interpolation, Sin(X)/X
interpolation, and polynomial interpolation. The zero-crossing
times 44 are used to determine the period of the waveform, which
together with the digitizer sample rate can be used to calculate
the IF. The technique is applied repeatedly to all RF pulses. The
final IF frequency measurement is the average of all the measured
results 46, after rejection of outliers (preferably between 1 and 2
standard deviations) 48. The actual RF frequency is the sum of the
measured IF frequency and the control frequency of the local
oscillator 24.
FIG. 4 illustrates an alternate embodiment of the invention using
direct Sin(X)/X interpolation of the acquired data to determine
zero-crossing points. The acquired data of FIG. 2 serves as the
input to the pulse frequency measurement algorithm. The method
interpolates the signal by first extracting pulse points and the
baseline 46. The baseline is extracted and subtracted from the
pulse points in order to remove any DC component from the pulse
sine wave, thereby improving accuracy of the zero-crossings. The
method then locates initial points before and after each zero
crossing 48 within a time interval. The time interval is then
subdivided into ten points 50 and interpolated using Sin(X)/X to
find points before and after a zero crossing within a subdivision
52. An iterative search algorithm finds the zero-crossing point to
an arbitrary accuracy of between 1/100 to about 1/1,000,000 of the
sample interval of the digitizer 54. It is understood that any
interval can be used, however, the smaller the interval, the
greater the accuracy. Once a point is determined to be within the
interval, the point is added to a list 56 of zero crossings. The
technique is applied repeatedly to all RF pulses. The zero-crossing
times are used to determine the period of the waveform, which
together with the sample rate can be used to calculate the IF 58.
The final IF frequency measurement is the average of all the
measured results 60, after rejection of outliers (preferably
between 1 and 2 standard deviations) 62. The actual RF frequency is
the sum of the measured IF frequency and the local oscillator 24
control frequency of the down converter.
The practical importance of the invention will be realized when it
is understood that the method described can significantly improve
the accuracy of measuring the frequency of narrow pulse widths in a
cost efficient manner, in real time and in a noisy environment.
It will be understood that the particular embodiments described
above are only illustrative of the principles of the present
invention, and that various modifications could be made by those
skilled in the art without departing from the scope and spirit of
the present invention, which is limited only by the claims that
follow.
* * * * *